Dorsoventral patterning of the C. elegans postembryonic mesoderm requires both LIN-12/Notch and TGFβ signaling

The C. elegans postembryonic mesodermal lineage arises from a single cell M, which generates distinct dorsal and ventral cell types. We have previously shown that mutations in the Schnurri homolog sma-9 cause ventralization of the M lineage and that wild-type SMA-9 antagonizes the Sma/Mab TGFβ pathway to promote dorsal M lineage fates [Foehr, M.L., Lindy, A.S., Fairbank, R.C., Amin, N.M., Xu, M., Yanowitz, J., Fire, A.Z., Liu, J., 2006. An antagonistic role for the C. elegans Schnurri homolog SMA-9 in modulating TGFbeta signaling during mesodermal patterning. Development 133, 2887–2896]. Interestingly, loss-of-function mutations in the Notch receptor lin-12 cause dorsalization of the M lineage [Greenwald, I.S., Sternberg, P.W., Horvitz, H.R., 1983. The lin-12 locus specifies cell fates in Caenorhabditis elegans. Cell 34, 435–444]. We have found that although LIN-12 protein is present in both the dorsal and ventral M lineage cells, its ligands LAG-2 and APX-1 are asymmetrically localized in cells adjacent to ventral M-derived cells, and may function redundantly in promoting ventral M lineage fates. To investigate how LIN-12/Notch signaling interacts with SMA-9 and Sma/Mab TGFβ signaling in regulating M lineage patterning, we generated double and triple mutant combinations among lin-12, sma-9 and dbl-1 (the ligand for the Sma/Mab TGFβ pathway) and examined their M lineage phenotypes. Our results suggest that the LIN-12/Notch pathway and the Sma/Mab TGFβ pathway function independently in regulating dorsoventral patterning of the M lineage, with LIN-12/Notch required for ventral M lineage fates, and SMA-9 antagonism of TGFβ signaling required for dorsal M lineage fates. Our work provides a model for how combined Notch and TGFβ signaling regulates the developmental potential of two equipotent cells along the dorsoventral axis.